The semiconductor furnace plays an important role in the semiconductor manufacturing process. Common processes such as oxidation, diffusion, chemical vapor deposition, and annealing are performed in a furnace. A typical type of furnace used is the so called "lamp annealing" furnace. The lamp annealing furnace has the advantage that a plurality of wafers can be treated and the process temperature can be precisely controlled. However, the lamp annealing furnace does not currently provide uniform temperature distribution over the entire wafer surface. Another popular type of furnace used in the semiconductor manufacture is the "hot wall" furnace. There are two types of "hot wall" furnace. One is the hot wall horizontal diffusion furnace which is capable of controlling temperature over the range of 300.degree.-1200.degree. C. to an accuracy of 0.5 or -0.5.degree. C. over a length of up to 40 inches.
Another type is the vertical furnace system. The major advantage of the vertical furnace system offers over a conventional system are (1) no cantilever or soft-landing is required since the wafers are held in a quartz boat which does not touch the process tube walls (2) the wafers can be loaded and unloaded automatically. (3) batch processes can be run in the same process tube. (4) temperature is more stable than other type.
In VLSI fabrication, the accurate control of layer thickness and reduction of defects are more important than ever. Many semiconductor processes are performed in a furnace such as oxidation, thermal annealing, deposition, etc . . . . Typically, one or more kinds of gases have to be injected into a furnace system. For example, in a thermal oxidation process, oxygen is injected into the furnace so that the wafer is kept in an oxygen ambient.
In addition, some of the processes are related to the reaction of oxygen and C.sub.2 H.sub.2 Cl.sub.2. It is a relatively simple reaction as shown below: ##STR1## The gases that are generated during the process are HCl and CO.sub.2. HCl is a poisonous gas that is extremely harmful to humans.
FIG. 1 shows in schematic form a conventional furnace system. A furnace tube 2 is used for process reaction that is set on a bottom board 4. The bottom board is used for setting a boat. Typically, wafers are transferred into the boat. A gas injection pipe 6 is connected to the furnace tube 2. The unreacted input gas is let through the pipe 6 into the furnace tube 2. An exhaust pipe 8 having a valve 10 is connected to the furnace tube 2 via a process gas exhaust pipe 26. The waste gas that is generated by a process is exhausted via the exhaust pipe 8 to an exhaust scrubber 12.
An automatic pressure controller (APC) 14 is set midway of the exhaust pipe 8 between the furnace tube 2 and the exhaust scrubber 12 and is used for maintaining and controlling the air pressure at a predetermined level inside the exhaust pipe 8 and the furnace tube 2. Valve 10 that is connected to the APC 14 is used to adjust the flow rate of the exhaust in the exhaust pipe 8. The APC 14 includes a sensor for monitoring the pressure inside the pipe 8. The mechanism of the APC 14 is controlled in accordance with the signal detected by the pressure sensor. For example, a value of air pressure is reached higher than -10 mm H.sub.2 O, the valve 10 will be responsive to the pressure and automatically opened. When the pressure approaches lower than -10 mm H.sub.2 O, the valve 10 will be automatically closed the opening of the valve 10 a little in order to release exhaust out of the furnace tube 2 and keep the air pressure at the valve of -10 mm H.sub.2 O.
In general, if the pressure inside the furnace tube 2 is higher than the pressure outside the furnace, then gas inside the furnace 2 will tend to leak out of the furnace tube 2 via the seam between the furnace tube 2 and the bottom board 4. Therefore, a vacuum seal 16 is attached to the bottom board 4 to prevent the gas from leaking. Typically, the vacuum seal 16 is connected to the exhaust pipe 8 at a node 17 via a vacuum seal pipe 20. A valve 10 is located midway between the node 17 and the bottom board 4 on the pipe 20. Another element of the furnace system is a water collector 22 located between the node 18 and a drain assembly 24. The water collector 22 is used to collect condensed water. The water also serves the dual purpose of being a pressure barrier. When water is collected by the water collector 22, gas cannot be exhausted from the drain assembly 24. A valve 10 is also typically located between the water collector 22 and the drain assembly 24.
In order to prevent the gas inside the furnace from leaking, the process gas exhaust pipe 26 has to provide a negative pressure into the furnace. However, the air pressure close to the process gas exhaust pipe 26 is very unstable due to the unstable air pressure inside the exhaust pipe 8. The variation of the air pressure is in the range of -10 mm H.sub.2 O to 0 mm H.sub.2 O. Therefore, the furnace pressure near the vacuum seal 16 is unstable between negative pressure and positive pressure. If the pressure reaches the positive pressure, then the gas will leak out of the furnace.
Another type of prior art furnace system that is used to prevent gas leakage is shown in FIG. 2. The furnace system includes a furnace tube 2a set on a bottom board 4a. A gas injection pipe 6a is connected to the furnace tube 2a for injecting gas into the furnace tube 2a. An automatic pressure controller (APC) 14a is located midway between the furnace tube 2a and the exhaust scrubber 12a via exhaust pipe 8a for maintaining the air pressure inside the furnace tube 2a. A valve 10a that is connected to the APC 14a to adjust the flow rate of the exhaust in the exhaust pipe 8a. A water collector 22a is located between the APC 14a and a drain assembly 24a. Similarly, the water collector 22a is used to collect condensed water that is used as a barrier. An N.sub.2 seal 16a is connected to the bottom board 4a for providing positive gas pressure into the furnace tube 2a in order to prevent gas leakage. Typically, the N.sub.2 seal 16a is connected to a port 4b which is attached to the bottom board 4a. A flow meter 16b is connected to the N.sub.2 seal 16a for controlling the flow rate of the N.sub.2.
The N.sub.2 is injected into the furnace tube 2a to generate a positive pressure near the bottom board 4a. This generally prevents the gas inside the furnace from leaking. Unfortunately, the injected N.sub.2 is forced into the furnace only via the port 4b. This often times allows gas to still leak out of the furnace tube 2a from the seam disposed away from the port 4b.
Another modification of the furnace system with an N.sub.2 seal is shown in FIG. 3. As can be seen, an open valve 26 is connected to the exhaust pipe 8a at node 18a. The open valve 26 will compensate for the pressure of process exhaust. Thus, air pressure variation will be reduced to the range of -9 mm H.sub.2 O to -11 mm H.sub.2 O. Although the modification can keep the gas exhaust pressure at the stable negative pressure. However, this design can not prevent the gas from leaking completely.